1. Field of the Invention
This invention relates to thin film metal alloy magnetic recording disks for horizontal recording, and in particular to such disks in which an alloy of cobalt, platinum and chromium forms the magnetic layer.
2. Description of the Prior Art
Cobalt-based alloys which are known as capable for use in horizontal magnetic recording media include cobalt-nickel (CoNi), cobalt-rhenium (CoRe), cobalt-palladium (CoPd), cobalt-chromium (CoCr) and cobalt-platinum (CoPt). In such media, the hexagonal close packed (HCP) crystalline structure of the cobalt alloy is formed on the substrate, or on an intermediate underlayer, so that the C-axis, i.e., the [002] axis, of the cobalt alloy film is either in the plane of the film or has a component in the plane of the film.
While CoCr is known primarily for its use in perpendicular magnetic recording media, a CoCr magnetic layer may exhibit horizontal magnetic anisotropy in the presence of a specific underlayer. For example, a thin film disk for horizontal recording with a composite magnetic layer of CoCr and Cr formed over an underlayer of nickel-vanadium is described in U.S. Pat. No. 4,552,820. However, CoCr has several disadvantages as a magnetic layer for horizontal recording. In order to assure horizontal magnetic anisotropy a relatively thick, e.g. 2000-3000 Angstroms, underlayer is required. In addition, because the horizontal coercivity of a CoCr layer decreases with increasing thickness of the layer, the maximum horizontal coercivity is approximately in the range of 600-800 Oersteds (Oe).
The coercivity and other properties of CoPt films have been reported by Opfer, et al., in "Thin Film Memory Disc Development", Hewlett-Packard Journal, November 1985, pp. 4-10; and by Aboaf, et al., in "Magnetic Properties and Structure of Co-Pt Thin Films", IEEE Trans. on Magnetics, MAG-19, 1514 (1983). A CoPt thin film magnetic recording medium with between 10-30 atomic percent (at.%) platinum (Pt) is described in U.S. Pat. No. 4,438,066, which is assigned to the same assignee as this application. While CoPt media exhibit desirable magnetic properties, the CoPt magnetic layer is highly susceptible to oxidation or corrosion. Thus, as described in European patent application 145157, published June 19, 1985, and assigned to Hewlett-Packard Company, it is necessary to apply a barrier layer of chromium, titanium or other suitable material over the CoPt magnetic layer.
A thin film disk with a cobalt-platinum-chromium (CoPtCr) magnetic layer, in which chromium (Cr) comprises between 1-17 at.%, is described in Japanese patent application Kokai No. 59-88806, published May 22, 1984. The CoPtCr magnetic layer is described as being sputter-deposited directly onto an aluminum substrate with a nickel-phosphorous coating and as having improved corrosion resistance over prior magnetic layer alloy compositions.
European patent application 140513, published May 8, 1985, and assigned to the same assignee as this application, describes on pages 9-12 a horizontal recording structure comprising a magnetic layer of a CoCrZ ternary alloy formed on a nonmagnetic underlayer of a CoCrX ternary alloy. The elements Z and X may be chosen from a long list of elements, including Pt, and may be present in a wide range of percentage compositions. The magnetic layer may not be deposited directly on the substrate but must be formed on the specific ternary alloy underlayer in order to exhibit in-plane C-axis orientation.
One of the problems with thin film metal alloy media is that the intrinsic media noise increases with increasing linear recording density. Media noise arises from fluctuations in the zig-zag shaped magnetic transitions and results in random shifts of the readback signal peaks. These random shifts are referred to as "peak jitter" or "timer jitter". Thus, the higher the media noise, the higher the bit error rate. It is therefore desirable to develop a thin film metal alloy media which generates noise below a maximum acceptable level in order that data can be recorded at maximum linear density. The effect of intrinsic media noise, as measured by peak jitter and media signal-to-noise ratio (SNR), on the bit error rate in magnetic recording systems is described by Katz, et al., in "Effect of Bitshift Distribution on Error Rate in Magnetic Recording", IEEE Trans. on Magnetics, Vol. MAG-15, pp. 1050-1053, 1979. The measurement of media SNR is described by Belk, et al., in "Measurement of the Intrinsic Signal-to-Noise Ratio for High Performance Rigid Recording Media", J. Appl. Physics, 59(2), Jan. 15, 1986, pp. 557-562.